The present invention relates generally to a reciprocating motor with a uni-directional fluid flow path. The present device may be employed to convert fluid energy into useful mechanical work for any machine, such as a reciprocating piston pump. The present device is particularly advantageous for applications such as cryogenic pumps where the continuous uni-directional flow of fluid reduces the effect of heat transfer between the fluid within the reciprocating motor and the cryogenic apparatus.
Conventional double-acting reciprocating motors use differential fluid pressure applied to a piston to cause reciprocating movement of the piston within a motor cylinder. Chambers on either side of the piston are equipped with respective fluid inlets and outlets that are controlled by external valves.
The piston moves to expand the volume of a first chamber by opening the inlet valve and closing the outlet valve associated with the first chamber while closing the inlet valve and opening the outlet valve associated with the second chamber on the opposite side of the piston. High-pressure fluid enters the first chamber through the open inlet valve while fluid is drained from the second chamber through the open outlet valve.
To move the piston in the opposite direction, the valve settings are reversed so that high-pressure fluid fills the second chamber and fluid is drained from the first chamber.
This type of reciprocating motor is known as a xe2x80x9cdouble-actingxe2x80x9d motor because fluid pressure is employed to move the piston in both directions and the piston rod extending from the reciprocating motor can perform mechanical work when traveling in both directions. A double-acting reciprocating motor is needed to drive a double-acting cryogenic pump that is designed to compress a cryogen with each piston stroke. That is, the pump piston compresses cryogen in both directions.
U.S. Pat. No. 4,458,579 (the ""579 patent) discloses a motor for actuating a downhole pump in an oil well. The motor employs fluid pressure to raise the piston. At the top of the piston stroke a valve opens to allow the fluid to flow through the piston. The ""579 patent discloses a motor with uni-directional fluid flow, but the motor is a single-acting motor that relies upon the force of gravity for downward movement of the piston. The motor has no valve at the fluid outlet for allowing fluid pressure to build in the cylinder space above the piston during the down-stroke.
U.S. Pat. No. 5,341,723 (the ""723 patent) discloses a reciprocating air motor with a uni-directional air flow through the motor cylinder. The ""723 patent discloses an internal venting arrangement whereby at the end of the piston stroke a groove in the cylinder wall allows the pressurized air to enter an internal chamber within the piston to open a valve to vent the pressurized air through the piston. However, like the ""579 patent, the ""723 patent does not disclose a double-acting reciprocating motor in that the pressurized air that passes through the piston is simply vented and a spring is employed to push the piston back to the starting position.
U.S. Pat. No. 5,203,251 (the ""251 patent) discloses an air motor that has an air inlet and outlet on the same side of the piston. The air exits the motor through a bore formed in the piston rod. This arrangement may be suitable for air motors where the air is typically vented after exiting the motor. However, removing the fluid through the piston rod results in a more complicated arrangement in a closed loop system, which is typically the case when the fluid is a hydraulic oil or other liquid. When a high pressure fluid is employed, for example, for applications such as driving cryogenic pumps, an essentially incompressible liquid is typically employed instead of a gaseous fluid, such as air. Discharging the air through the piston rod, as disclosed by the ""251 patent, also increases the time that the fluid is within the motor assembly and directs the fluid back to the same side as the inlet before the fluid is ultimately recovered in a closed-loop system. If this arrangement is employed for driving a cryogenic pump, the fluid would be directed back to the xe2x80x9ccoldxe2x80x9d side before exiting the motor.
For cryogenic applications, the fluid is typically a liquid such as a hydraulic oil that is virtually incompressible and that also helps to lubricate the piston and cylinder. A particular problem with known double-acting reciprocating motors, which are employed to drive cryogenic pumps, is that there is a potential for the liquid within the motor cylinder nearest the cryogenic pump to become frozen. The problem is exacerbated if the same liquid is repeatedly returned to the xe2x80x9ccoldxe2x80x9d side of the reciprocating motor without being directed back to the fluid reservoir or to the xe2x80x9cwarmxe2x80x9d side of the motor that is further from the cryogenic pump. Thermal insulation is typically provided to shield the liquid from the cooling effect of the cryogenic pump. However, thermal insulation interposed between the cryogenic pump and the reciprocating motor adds to the weight, bulk and overall length of the pump and motor assembly. Furthermore, it is difficult to completely eliminate heat transfer because the piston rod assembly acts as a thermal conductor between the reciprocating motor and the cryogenic apparatus.
If actuation liquid is cooled so that it freezes within the reciprocating motor cylinder, severe damage may be caused to the motor and/or piston rod.
An objective of the present device is to provide a differential pressure, reciprocating motor with a uni-directional fluid flow path for applications that employ a double-acting motor. A particularly suitable application is for driving a cryogenic pump because the uni-directional flow path helps to reduce the effects of heat transfer between the cryogenic pump and the reciprocating motor. With a uni-directional flow path, the fluid flows through the reciprocating motor in one direction, flowing for example, from a high pressure fluid supply to the piston cylinder on a first side of the motor piston, then to a second side of the motor piston (opposite to the first side). The fluid is finally drained from the second side of the motor piston and returned to a reservoir.
A double-acting reciprocating motor with a uni-directional flow path is provided that comprises:
a housing having a hollow cylinder disposed between a cylinder head and a cylinder base;
a piston disposed within the cylinder between the cylinder head and cylinder base, the piston having a first pressure surface area and a second pressure surface area opposite to and larger than the first pressure surface area;
a piston rod operatively associated with the piston and extending from the piston through the cylinder base;
a fluid inlet for directing fluid to a first chamber within the cylinder associated with the first surface area;
a fluid outlet for draining fluid from a second chamber within the cylinder associated with the second surface area;
a fluid passageway disposed within the piston, the fluid passageway fluidly connecting the first chamber to the second chamber;
a pass-through valve for selectively opening and closing the fluid passageway; and
an outlet valve that is openable for draining fluid from the outlet when the pass-through valve is in the closed position.
In a preferred embodiment the fluid is a liquid and the reciprocating motor is for driving a double-acting cryogenic pump.
In one embodiment the pass-through valve comprises a movable plunger disposed within a bore formed in the body of the pass-through valve, wherein:
the bore has a longitudinal axis that is parallel to the longitudinal axis of the cylinder;
the plunger is movable to reciprocate within the bore; and
the pass-through valve is actuated to switch between open and closed positions by an end of the plunger contacting a surface of the housing when the piston approaches one of the cylinder base and the cylinder head.
The outlet valve may comprise, for example, a plunger movable within a bore provided in the outlet valve; the plunger comprises a sealing surface that may be urged against a valve seat to close the outlet valve and lifted away from the seat to open the outlet valve. The plunger may further comprise a valve stem attached thereto for actuating the outlet valve. The outlet valve is automatically actuated by contact between the piston and the valve stem when the piston approaches one of the cylinder head and the cylinder base.
One end of the stem attached to the outlet valve plunger may be disposed within a well formed within the piston. In such an arrangement, the piston further comprises an actuating plate that contacts an enlarged end portion of the valve stem to lift the plunger from the valve seat when the piston approaches the cylinder base. The plunger sealing surface may be urged against the valve seat by the piston contacting the valve stem or the plunger. For example, as the piston approaches the cylinder head, the piston may contact the valve stem or plunger directly and push the plunger into the seated position. In another arrangement, as the piston approaches the cylinder head, the bottom of the well may contact the end of the valve stem disposed within the well and thus urge the plunger into the seated position.
In another preferred embodiment, the pass-through valve and the outlet valve are combined in an integrated valve assembly. For example, an integrated valve assembly may comprise:
a tubular valve body associated in fixed relationship with the cylinder head;
a tubular plunger disposed within the tubular valve body with a closed end facing the cylinder head and an open end fluidly connected to the first chamber wherein the tubular plunger is movable within the valve body;
a spring for urging the tubular plunger between a first position and a second position wherein the spring urges the tubular plunger into the first position when the piston approaches the cylinder base and into the second position when the piston approaches the cylinder head;
wherein when the tubular plunger is in the first position, openings formed in the tubular valve body allow fluid to drain from the second chamber through an outlet port and openings formed in the tubular plunger are covered by a portion of the interior wall of the tubular valve body; and
wherein when the tubular plunger is in the second position, the valve body openings and the plunger openings are aligned whereby fluid is able to flow from the first chamber through the interior of the tubular plunger and through the aligned openings into the second chamber and the closed end of the plunger prevents fluid from flowing out from the second chamber through the outlet.
A method is also provided for operating a double-acting reciprocating motor comprising a movable piston disposed within a cylinder between a cylinder head and a cylinder base. The motor comprises a first variable volume chamber formed between the cylinder base and a first piston pressure surface, and a second variable volume chamber formed between the cylinder head and a second piston pressure surface. The second piston pressure surface is larger than the first piston pressure surface. A pass-through valve is operable to allow fluid to flow from the first chamber to the second chamber. An outlet valve is operable to drain fluid from the second chamber. The method of operating such a device comprises:
introducing the fluid through an inlet port into the first chamber to cause reciprocating motion of the piston;
closing the pass-through valve and opening the outlet valve when the piston approaches the cylinder base so that fluid pressure within the first chamber causes the piston to move towards the cylinder head while fluid is drained from the second chamber through the outlet valve;
opening the pass-through valve and closing the outlet valve when the piston approaches the cylinder head so that fluid pressure within the second chamber causes the piston to move towards the cylinder base.
In a preferred method the fluid employed for applying pressure to the piston is a liquid. The method preferably further comprises introducing the fluid through the inlet port that is formed in the cylinder base and draining the fluid through the outlet valve, which comprises an outlet port formed in the cylinder head. In such an arrangement the fluid enters one end of the motor and exits the motor from an opposite end. This is an advantage where it is desirable to simplify the flow path of the fluid and where it is desirable to reduce the heat transfer between the fluid and apparatus driven by the motor. For example, heat transfer is an important consideration when the motor is coupled to a cryogenic pump for driving a reciprocating pump piston.
Another advantage of the present motor is that it provides an arrangement that reduces the number of external connections and valves to operate the motor, compared to conventional differential piston double-acting reciprocating motors.